Load limiting device for a seat belt

ABSTRACT

A load limiting device is used with a vehicle seat belt during a crash to allow a limited and controlled forward motion of a vehicle occupant after the retractor has locked. The load limiting device decreases the forces exerted by the seat belt on the vehicle occupant&#39;s torso. The load limiting device has a container which defines a flow path for a fluid, with a restriction in the flow path. An actuator is located within the container and is movable within the container under the action of a load transmitted to the seat belt webbing from the vehicle occupant under extreme acceleration conditions, to move the fluid through the restriction in the flow path to dissipate a predetermined portion of the load on the webbing.

FIELD OF THE INVENTION

The present invention relates to a load limiting device for use with aseat belt.

BACKGROUND OF THE INVENTION

Modern seat belts comprise a length of webbing arranged to passdiagonally across the torso of a vehicle occupant and generally,horizontally across the hip region of the vehicle occupant (theso-called lap portion of the belt). This is known as a three point beltsystem. One end of the belt webbing is firmly attached to a structuralpart of the vehicle, such as the floor, and the other end is attached tothe spool of a retractor which itself is firmly attached to a structuralpart of the vehicle, usually the side, B-pillar. The retractorautomatically keeps any slack in the belt wound onto the spool and thuskeeps tension in the belt. Between the retractor and the other fixedpoint, a fastening element such as a metal tongue is fixed to the beltwith which it can be fastened into a buckle which itself is attached toa fixed part of the vehicle on the other side of the seat.

A clock spring in the retractor allows pay out of webbing under theinfluence of relatively gentle forwardly directed inertia of the vehicleoccupant, for example to allow for normal movement of the vehicleoccupant such as reaching forward to activate in-car controls, glovecompartments or door pockets.

In the event of a crash the sudden high forward momentum of the vehicleoccupant activates a crash sensor which locks the spool against rotationand prevents forward motion of the vehicle occupant to prevent himcolliding with the internal structure of the vehicle such as thesteering wheel, dashboard or windscreen.

However, it has been found in high velocity crashes that the suddenlocking of the seat belt can itself cause injury to the vehicle occupantdue to the sudden impact of the torso with the belt webbing.

In recent years it has been proposed to introduce a load limiting effectinto the seat belt system so as to allow a limited and controlledforward motion of the vehicle occupant after the retractor has locked.This decreases the forces exerted by the belt on the vehicle occupant'storso.

DISCUSSION OF THE PRIOR ART

Load limiting proposals are described in EP 0297537 wherein aplastically deformable member is used in the retractor, and particularlybetween the spool and inner most winding of the belt webbing.Alternative load limiting proposals are known in which crushablebushings or nuts or deformable torsion bars are placed in the retractorin the force path between the spool locking mechanism and belt webbing.These proposals are complex and expensive and require the retractor tobe specially designed and constructed to incorporate them.

SUMMARY OF THE INVENTION

The present invention proposes improved, simpler and more cost effectiveload limiting devices for seat belts.

According to one aspect of the present invention there is provided avehicle safety restraint comprising seat belt webbing, for restraining avehicle occupant, the webbing being attached to a structural part of thevehicle via a load limiting device, the load limiting device comprisinga container defining a flow path for a fluid, an actuator arrangedwithin the container and operable to act on the fluid, and a restrictionin the flow path of the fluid, wherein the actuator is movable withinthe container under the action of load transmitted to the webbing fromthe vehicle occupant under extreme acceleration or decelerationconditions, to move the fluid through the restriction in the flow paththereby to dissipate a predetermined portion of the load on the webbing.

According to one embodiment of the invention the actuator comprises apiston and the container comprises a cylinder, and either the piston orcylinder is attached to an end of the belt webbing and the other isattached to the structural part of the vehicle.

The restriction may comprise one or more holes in the piston itself sothat under pressure fluid is forced from one part of the cylinder, onthe high pressure side of the piston, to the other part of the cylinder,on the low pressure side. Alternatively, there is a feedback pipeexternal to the cylinder to feed fluid under pressure from the highpressure side of the piston to the low pressure side. This feedback pipemay have a further restriction or a valve. The valve may be adjustableto offer a predetermined resistance to fluid flow in dependence upon theforce damping characteristics required for the particular seat belt. Thevalve may be actively controllable, for example in response to a gaugeconstantly monitoring the pressure of the fluid in the cylinder. In thisway the load perceived by the vehicle occupant can be actively tailored,not only to the vehicle concerned, but also to the circumstances whichgenerated the load: the severity of the crash pulse, the weight of thevehicle occupant and the activation of a secondary restraint such as anairbag, or the occurrence of a second crash.

The piston may be arranged so that the cross sectional area which itpresents to the fluid is variable. For example, it can be formed of aseries of concentric tubes which are picked up by the core piston withincreasing belt payout as the load increases. This increases thepredetermined load at which the load limiting effect comes intooperation and thus increases the restraining effect of the belt withincreasing belt extension. This is particularly appropriate for example,in a severe crash and/or with a heavy vehicle occupant. In this case itwould be important to ensure that the occupant is stopped completelyfrom his forward motion before the end of the load limiting device isreached.

On the other hand, there are circumstances in which it is desirable todecrease the restraining effect as the crash pulse progresses. This canbe done by decreasing the predetermined load at which load limitingbecomes operational as the belt payout increases. For example, if anairbag is fitted to the vehicle, then that will take over therestraining effect and stopping the seat belt completely at an earlystage of the crash pulse is not important. In this case the piston canbe arranged to lose sections of its effective cross section as ittravels down the cylinder. The exact arrangement can thus be easilyadapted to obtain the required effect.

In an alternative embodiment the actuator is a rotary paddle or vanemember or an array of such members, mounted for rotation in a housingcontaining the fluid. This embodiment may, for example, take the generalform of an eccentric vane pump or a peristaltic pump or a swash platepump, though many other suitable forms of pump arrangement will beevident to those skilled in the art. In each case the fluid flow pathwithin the pump housing is arranged with one or more restrictions toabsorb some of the energy of a crash pulse.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried into effect, reference will now be made, by way ofexample, to the accompanying drawings.

FIG. 1 illustrates one embodiment of the present invention.

FIG. 2 is a graph illustrating force against belt extension for theembodiment of FIG. 1.

FIG. 3 is a perspective view of a second embodiment of the presentinvention.

FIGS. 4a, 4b and 4c are cross-sectional views of a part of theembodiment of FIG. 3.

FIG. 5 is a cross-sectional view of a development of the secondembodiment.

FIG. 6 is a transverse cross-sectional view of the embodiment of FIGS.3, 4 and 5.

FIG. 7 illustrates a further embodiment of the present invention.

FIG. 8 illustrates, in longitudinal cross-sectional view, anotherembodiment of the present invention.

FIG. 9 illustrates, again in cross-sectional view a further alternativeembodiment.

FIGS. 10a, 10b, 10c and 10d illustrate a further embodiment of thepresent invention wherein FIG. 10a is a perspective view and FIG. 10d isa graph showing force exerted by the safety restraint using the loadlimiter of FIG. 10a.

FIGS. 11a and 11b illustrate another embodiment of the presentinvention, in perspective view in FIG. 11a and in cross-sectional viewin FIG. 11b.

FIGS. 12a and 12b illustrate another embodiment, in perspective view inFIG. 12a and cross-sectional view in FIG. 12b.

FIG. 13 illustrates another embodiment, in perspective view in FIG. 13aand in fragmentary cross-sectional view in FIG. 13b.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a cylinder 1 with a fixing hole 2 at one end for connectionto a structural part of a vehicle, (not shown) either at the buckle endor at the sill end of a seat belt. Inside the cylinder 1 is a piston 3with at least one hole 4 therethrough. The piston 3 is connected by acable 5 to seat belt webbing 6 and thus the device of FIG. 1 provides aload limiting device between a structural part of the vehicle and theseat belt webbing of a three point system, either at the buckle end orthe sill end. Inside the cylinder 1 is a fluid 7 which may be a gas or aliquid or some other extrusible material such as a gel or a foamcomposition. The exact nature of this extrusible and/or compressiblematerial is chosen to suit the load limiting characteristics required ofthe load limiting device.

Towards one end of the cylinder a crush tube 8 is arranged coaxiallywith the cylinder. This crush tube is an optional feature but isincorporated to tune the load limiting effect to increase the energyabsorption over a particular range of belt extension, specifically nearto the end of travel of the piston. The force is controlled by the sizeof the hole or holes 4 in the piston 3. A spring 11 is also included tobias the piston in the direction of least extension, i.e. in the rearposition. The spring also increases the limiting load.

This effect can be seen in FIG. 2 where curve A indicates a highvelocity crash force extension curve relationship for thepiston-cylinder arrangement of FIG. 1 and line B indicates a lowvelocity crash force belt extension curve relationship. Line C indicatesthe crash pulse without any force limiting.

When a crash is indicated, a crash sensor signal causes the retractor tolock up against further webbing extension. The force felt by the vehicleoccupant rises relatively steeply in the first part of the curves A andB, mirroring the crash pulse C itself. There will be some forwardmovement of the vehicle occupant due to belt slack being taken up (filmspool effect). This allows a limited belt extension of the distance xafter the spool has locked. However when a predetermined level of loadis applied to the piston and the film spool effect is over, the inherentinertia of the system is overcome and the piston begins to move alongthe cylinder 1 applying a force to the fluid or gel 7 in the cylinder 1and forcing it through the holes 4 in the piston. This absorbs some ofthe load which would otherwise be felt by the vehicle occupant andallows a further webbing extension y, at a more constant load, asindicated by the relative plateaus in the graphs A and B. In theembodiment of graph A, the fluid compression characteristics exhibit agradient and thus the plateau between x and y is not so flat as forgraph B.

Towards the end of the cylinder the piston encounters the crush tube 8which slows its movement. The force felt by the vehicle occupant risesagain relatively sharply and then exhibits another plateau as indicatedbetween y and z in the graphs A and B when the crush tube 8 is almostcompletely crushed the force on the vehicle occupant will rise againsteeply at the z. However at this point the force of the crash isdissipating as can be seen by the crash pulse indicated by the line C.

Of course the device could be made such that the tube crush force islower than the fluid compression or extrusion force. In this case thegraphs A and/or B will be effectively the same shape but the plateaubetween x and y will be attributable to the crush tube rather than thefluid and that between y and z to the fluid rather than the crush tube.

FIG. 3 illustrates another embodiment in which the piston is constructedin solid form, without holes, but a feed back loop pipe 10 carries thefluid from one part to the other part of the cylinder as the piston ispulled along the cylinder under tone from the seat belt. A control valveor restriction 9 is optionally introduced into the feedback pipe 10 soas to effect further control and tuning of the system. The fluid may begas or liquid and the viscosity of the fluid together with size of thereturn tube, controls the force required.

In FIGS. 4a, 4b and 4c a variable size piston surface is used to tunethe system further. The arrangement is such that the piston surface areadecreases with increasing belt extension so that the limiting loaddecreases and more payout is allowed for less load, for example as thecrash pulse dissipates, or as a secondary restraint such as an airbagcomes into operation. This produces a more constant force on the vehicleoccupant.

The piston is formed with a small central circular part 3a attached to acable 5, with a series of concentric cylinders 3b, 3c and 3d fittingsnuggly inside each other. At the beginning of the crash pulse thecentral piston part 3a moves together with all the concentric cylinders,in the direction of arrow E and thus presents a maximum piston surfacearea to the extrudible/compressible fluid 7. This is shown in FIG. 4aand gives the maximum resistance to movement of the piston and thus theminimum load limiting effect. However, as the belt extension increases,the outer concentric cylinder 3d reaches the end of cylinder 1 and isstopped by it. The adjacent cylinder 3c slides past the outer one as thepull on the belt and thus on cable 5 is still sufficient to overcome thefriction between the concentric cylinders. However the effective pistonsurface area, as shown in FIG. 4b is much reduced and thus theresistance to piston movement through the fluid is decreased, allowing agreater webbing payout for a fixed load, i.e. the load limiting effectis decreased.

As the belt extension increases further each of the outer concentriccylinders 3b to 3d come to a stop at the end of cylinder 1 and only thecentral piston part 3a continues to move. However at this stage as shownin FIG. 4c an even smaller piston surface is presented and thus the loadlimiting effect is minimal.

The level of load limiting is a function of the square of the velocityof the crash. This characteristic of fluids is well known in standarddamping technology. Thus in a high speed crash the damping effect of thefluid filled load limiter is very much higher than the effect in a lowspeed crash. Obviously this is advantageous.

The embodiment of FIGS. 5 and 6 is a variation on the embodiment ofFIGS. 3 and 4a. b and c in which the concentric tubes 3a, 3b, 3c and 3dare held together by a spring 11. These concentric cylinder tubes eachhave three lugs so that they are held in the position shown in FIG. 5under the action of spring 11 under normal, non-load limiting,conditions. However, when the webbing 6 is put under load and the pistonis pulled in the direction E, the inner piston 3a picks up each of thetubes 3b, 3c and 3d in turn so that the rate of extension of the webbingis varied during the full stroke of the piston. In this arrangement theeffective surface area of the piston increases with increasing extensionof the webbing as the crash pulse develops and thus the piston movesmore fluid and requires more force. A load limiter of this type could beused to come into effect earlier in the crash pulse than the loadlimiter shown in FIGS. 4a to 4c where the surface area of the pistoneffectively decreases.

A valve 9 is incorporated in the feedback return pipe 10 to restrict theflow of fluid to increases or decrease the force. The valve could be preset in the factory or controlled according to crash conditions orvehicle occupant size or position. A pressure sensor 16 may be used tocollect data to control the valve.

In the embodiment of FIG. 7 the inside of the cylinder 1 is profiled togive a predetermined rate of webbing extension for a given load. Inparts, the cylinder bore widens allowing fluid to leak past the piston:the size of the leakage area controls the force. For example thecross-sectional area of the cylinder bore increases with increasing beltextension at the beginning of the stroke but decreases towards the endof the stroke. The particular profile will be chosen to suit the loadlimiting characteristics, the expected crash pulse and the reaction ofthe particular safety restraint involved.

The piston 3 also has an internal tapering profile to give additionalload limiting near the end of the stroke as the last bit of fluid 7 istrapped and compressed. The internal conical profile of the piston 3fits over the externally tapering position 12 at the other end of thecylinder 1.

The piston is biased by a spring 11 to its start position as shown inFIG. 7. After the initial extension of the webbing, because of theexpanding cross-sectional profile of the cylinder 1, there is an annulusaround the periphery of the piston 3 and the fluid 7 is forced throughthis annulus thus absorbing some of the energy, allowing webbingextension and limiting the load on the vehicle occupant.

FIG. 8 shows an embodiment in which the cable 5 is profiled to providethe appropriate load limiting characteristics. In the rest position noexhaust port exists between the cable and the end of the cylinder 1. Asthe piston moves in the direction indicated by arrow E the profiledcable 5 (which could also be a solid rod or shaft) first decreases incross-section and therefore effectively opens up an exhaust port 13.This allows outflow of fluid 7 that absorbs some of the energy of thecrash and limits the load felt by the vehicle occupant. The actual shapeof the cable or rod 5 is chosen to provide the appropriate load limitingcharacteristics.

A spring (not show) may be used in the cylinder 1 to bias the piston 3to the webbing retracted position. A fluid return pipe may be used tochannel fluid from one side of the piston to the other to provide loadlimiting without leakage of fluid out of the cylinder. In this case avariable diameter seal will be provided at the external wall of thecylinder to compensate for the profiling on the cable.

FIG. 9 illustrates another embodiment wherein a ball valve 14 biased bya spring is used to control exhaust vents 13. The piston 3 is biased inits rest position by a spring 11 inside the cylinder 1 and the fluid 7is on the spring side of the piston. A vent 15 is provided on the otherside of the piston 3 to avoid a vacuum being created. A sensor 16 isincorporated to sense the pressure of the fluid 7 and control the ballvalve 14 accordingly.

FIGS. 10a to 10c illustrate an embodiment using variable vent areas. Twosnugly fitting tubes are used, one sliding inside the other and onehaving an array of holes, the other having cut-out portions toselectively reveal or cover the holes depending upon the relativepositions of the tubes. FIG. 10a shows the assembled position and FIG.10b the disassembled device. The tubes 20, 21 are mounted in respectiveframes 22, 23, one of which is fastened to a structural part of thevehicle (not shown), and one of which is fastened to a part of the seatbelt webbing (not shown). The cylinder 21 has an array of vent holes 24whereas cylinder 20 has cutout portions in the form of control windows25. As the vent holes 24 move inside the outer tube, they allow fluid toescape.

FIG. 10d is a graph of force against belt extension and illustratesexposure of a particular array of vent holes at any particular beltextension. For example, with reference to FIG. 10c, in position 1, allvent holes are covered, in position 2 one vent hole is open, position 3,two vent holes are open etc. In this embodiment, using the particulararrangement of holes and exhaust vents illustrated in FIG. 10c, theforce felt by the vehicle occupant decreases with belt extension.

FIGS. 11 to 13 illustrate load limiting devices using rotary motion offluid. Like parts or referenced by like numerals. These load limitingdevices work in a similar manner to the linear versions described above.They can be mounted at either the sill end or buckle end of a belt or toany position at which a rotatable member is present or could beintroduced. For example a rod or spindle is mounted to a structural partof the vehicle and a few turns of webbing wound around it with the loadlimiter attached to the rod or spindle. Alternatively the load limitercould be connected to the retractor spool but in this case a clutcharrangement will be used to selectively connect the load limiter only incrash conditions.

In each of the embodiments of FIGS. 11 to 13 seat belt webbing 6 iswound on a rod or spool 30 which is mounted for rotation a frame 31(which may be a retractor frame). A load limiting device 40 is mountedat the spindle of the spool 30. The load limiting device is a fluidfilled pump actuated by rotation of the spool spindle so that the fluidin the pump is driven through restrictions and/or compressed and absorbssome of the energy of the crash.

In the embodiment of FIGS. 11a and 11b the pump is an eccentric vanepump which has a rotor 41 attached to a spool spindle (not shown) andfour rotating telescopic blades 42 mounted in a housing 43 defining achamber filled with fluid 7. The blades 42 keep in constant contact withthe inside wall of the housing 43 to maintain a good fluid seal. Thehousing 43 is eccentrically arranged compared to the spool spindle andthe rotor 41. As the blades 42 rotate (counter clockwise in the exampleshown by the arrow in FIG. 11b) the eccentricity of the housing causesthe effective chamber volumes between respective vanes to change insize; decreasing as the vanes 42 approach the upper part of the chamberand increasing as they approach the lower part. Thus the fluid is forcedalong an additional passage 44 and through a restrictor 45 (which couldbe programmable) arranged in this additional fluid passage 44.

In the embodiment of FIGS. 11a and 12b the pump is a peristaltic pumpbut the principle is similar. This embodiment comprises four rollers 47which rotate forcing fluid 7 around the flexible channel 48 supportedagainst the pump outer casing 49, and which has a soft inner wall 50. Atthe bottom of this channel 48 the fluid is forced through a restriction45. The size of the restriction controls the force applied to thewebbing.

The embodiment shown in FIGS. 13a and 13b comprises a swash plate pump.The spool spindle has a swash plate 54 and an eccentric cam plate 50which drives pistons 51 to pump fluid 7 through passages 52. Ball valves53, 60 are disposed in the passages 52 to restrict flow and thus absorbenergy. The pistons 51 are biased to their return positions by springs55 and ball valves 53, 60 have springs 56, 61 to control, the pressurerequired to force fluid past them. There are four sets of pistons andball valves; i.e. four sets of pumps and valves in the embodiment shownin FIG. 13a.

FIG. 13b illustrates one pump in more detail. As the swash plate 54rotates, the eccentric cam plate 50 rotates and piston 51 is pushed inagainst the action of spring 55. Movement of the piston forces fluidpast the ball valve 53, against the action of the spring 56, into fluidstorage chamber 57. As the piston returns to its original position itcreates a negative fluid pressure opening another ball valve 60 againstaction of another spring 61 and allows fluid in behind the piston 51.The cycle continues in this way and energy is absorbed by the fluidbeing forced through the restrictions of the ball valves.

From the foregoing, it will be apparent to those skilled in the art thatmodifications may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not intended to be limitedexcept as may be made necessary by the appended claims.

We claim:
 1. A load limiting device for use with a vehicle safetyrestraint having seat belt webbing attached to a structural part of avehicle, the load limiting device comprising a cylinder defining a flowpath for a fluid, a piston arranged within the cylinder and operable toact on the fluid, a restriction in the flow path of the fluid, a springinterposed between the piston and an end of said cylinder in thedirection that the piston will move in the event of a crash, and a crushtube arranged inside said cylinder and coaxial therewith interposedbetween the piston and said end of said cylinder in the direction thatthe piston will move in the event of a crash, wherein the piston ismovable within the cylinder under the action of a load transmitted tothe webbing from a vehicle occupant, to move the fluid through therestriction in the flow path and compress said spring and deform saidcrush tube to dissipate a portion of the load on the webbing.
 2. Theload limiting device of claim 1 wherein either the piston or cylinder isattached to an end of the belt webbing and the other is attached to thestructural part of the vehicle.
 3. The load limiting device of claim 2wherein the restriction comprises one or more holes in the piston sothat under pressure fluid is forced from one part of the cylinder, on ahigh pressure side of the piston, to another part of the cylinder, on alow pressure side of the piston.
 4. The load limiting device of claim 1wherein the force required to crush the crash tube is greater than theforce required to extrude the fluid through the restriction means. 5.The load limiting device of claim 1 wherein the force required to crushthe crash tube is smaller than the force required to extrude the fluidthrough the restriction means.